Ferroelectric liquid crystal (FLC) substances have been widely used as electro-optic modulators in applications such as flat panel displays, spatial light modulators, and specialized optical image processors, where switching times on the order of microseconds are needed. They are generally fabricated into surface-stabilization structures, so-called SSFLC (surface stabilized ferroelectric liquid crystal), for binary operation. (N. A. Clark et al., "Submicrosecond bistable electro-optic switching in liquid crystals," Appl. Phys. Lett. 36(11), p.# 899, 1980.).
A typical SSFLC modulator is made by the following process. Transparent electrode (indium-tin-oxide) coated glass substrates are generally used for the cell walls. They are spin-coated with alignment material, for example nylon or polyimide, and then rubbed with silk to form an alignment layer. The two substrates are brought together with the rubbing directions parallel or anti-parallel to each other. The cell thickness is kept much smaller than the pitch length of the liquid crystal material such that the liquid crystal helix is suppressed by the cell walls (glass substrates). The assembly creates a "chevron" structure when parallel rubbing is applied or a "quasi-bookshelf" structure when anti-parallel rubbing is applied to the substrates.
In these structures, liquid crystal molecules are switched between two states when positive and negative electrical fields are applied to them. This is due to the interaction between the applied electric field and the spontaneous polarization of the molecules. By selecting a ferroelectric liquid crystal material that has an angle of 45.degree. between the two states, the modulator becomes a switchable waveplate. By further designing the thickness of the liquid crystal cell such that .DELTA.n d=.lambda./2, the modulator acts as a switchable half-wave plate that can rotate the input linear polarization by 0.degree. or 90.degree.. The parameters .DELTA.n and d are the optical birefringence and the thickness of the liquid crystal material, respectively, and .lambda. is the operating wavelength. An input optical signal with its linear polarization aligned to one of the liquid crystal states experiences no polarization modulation. (The optical signal experiences only a phase delay.) When the molecules are switched to the opposite state at 45.degree., the polarization of the optical signal is rotated 90.degree.. A simple on/off switch can be constructed based on this ferroelectric liquid crystal modulator when two polarizers are added to the input and output of the device.
In the SSFLC mode, because the helix of the liquid crystal molecules is suppressed by the surface anchoring energy provided by the alignment layer, no analog modulation is allowed. The device operates in a binary switching mode.
Another type of ferroelectric liquid crystal modulator based on a twisted structure was disclosed by Patel, in U.S. Pat. No. 5,172,257. In this type of twisted ferroelectric liquid crystal modulator, the glass substrates are strongly rubbed or "buffed" at an angle of 90.degree. relatively to each other. Ferroelectric liquid crystals with a large tilt angle, about 90.degree., are used to fill the cell gap. Because of the strong buffing, liquid crystal molecules adjacent to both cell walls align to the buffing directions. The smectic layer of the liquid crystal is formed with its layer normal laid at 45.degree. relative to the two rubbing directions. Without the electrical field, the twisted structure waveguides the polarization of the input light to rotate the polarization by 90.degree.. With application of electrical fields, the waveguiding effect is distorted and the polarization is rotated partially. With the modulator sandwiched in between two crossed or parallel polarizers, analog intensity modulation can be obtained.
In this analog modulation case, the device is modulated between 0 and V or 0 and -V voltage states. When application voltage V&gt;V.sub.sat is used (where V.sub.sat is the saturation voltage) the molecules are switched at 0.degree. and 90.degree. positions (because a 45.degree. tilt angle material is used). With parallel or vertical polarization input to the modulator, only phase modulation (from n.sub.e d/.lambda. to n.sub.o d/.lambda., or vice versa, (where n.sub.o is the ordinary refractive index of the liquid crystal and n.sub.e is the so-called extra-ordinary refractive index) is obtained and no intensity modulation results.